This invention relates to the preparation of glasses, and, in particular, relates to the rapid preparation of glasses with high optical qualities.
Considerable effort has been expended to develop heavy metal halide glasses as a viable family of infrared-transparent optical materials for application as fiber waveguides and as bulk optical components. The specific characteristics of these materials and their advantages in terms of optical performance over silica and other oxide-based glasses are well known.
The heavy metal halide glasses include the so-called fluorozirconate and fluorohafnate glasses as well as glasses in which such atomic species as lead, cadmium, thorium, zinc, uranium, ytterbium, lutertium, gadolinium, cesium, rubidium, sodium, potassium, stroutium, calcium, magnesium, lithium yttrium, gallium, bismuth or beryllium, are either partially or completely substituted for the major constituents zirconium, hafnium, barium and the minor constituents lanthanum and aluminum in the fluorozirconate and fluorohafnate glassmaking formulations.
Because of fundamental physio-chemical differences, these halide glasses are not as easily formed into bulk components (e.g. plates, discs, rods) as are many multi-component glasses. For example, the viscosity (0.1-1.0 poise) of most heavy metal fluoride melts near the liquidus (800.degree.-1OOO.degree. C.) is similar to that of water. This high fluidity, coupled with the proximity of the glass-transition (Tg.about.320.degree. C.) and crystallization temperatures (T.sub.x .about.400.degree. C.) in many HMFG compositions, translates into an oftentimes marked tendency for melts to devitrify or crystallize upon cooling. In addition, the presence of specific impurities such as hydroxyl species and trace oxides may be instrumental in the nucleation and growth of crystallites.
Standard heating and cooling or quenching practices result in undesired compositional shifting within the phase diagram due to loss of one or more volatile constituents from the melt during prolonged heating which increases the possibility of devitrification or spontaneous crystallite nucleation as the melt cools to from a glass.
These problems are often exacerbated by the melting techniques traditionally utilized to prepare glasses which involve the use of oxide starting materials converted in situ to fluorides via heating with ammonium bifluoride, NH.sub.4 HF.sub.2. However, the present teaching does not preclude the use of oxides, oxyhalides, or fluorides in combination with fluorinating or reactive agents such as NH.sub.4 HF.sub.2, HF, F.sub.2, NF.sub.3, BF.sub.3, SF.sub.6, SF.sub.4, CF.sub.4 and other fluorocarbon gases, etc., nor does it preclude the use of premelted multicomponent fluoride combinations such as cullet or devitrite.
Prior processes also use premelted cullets which required storage, transport, breakup to appropriate size for charging the melting vessel and exposure to water in the environment.
Prior methods also heated the melts for a long period to provide homogenization of the components. Besides being totally unnecessary in the case of multicomponent fluoride halide glasses, this practice results in altered composition due to escape of volatiles as well as contamination from the sublimation and reaction products of said volatiles. Extended holding of temperature above 800.degree. C. particularly should be avoided in the case of fluorozirconate glass, for example, because the vapor pressure of the major (about 57 percent by weight) constituent, zirconium tetrafluoride, ZrF.sub.4, is one atmosphere at 800.degree. C. Its loss is readily observable as voluminous fuming and deposition of crystalline zirconium fluoride sublimate on the melt container supports, furnace components and viewports. It is, therefore, desirable to limit the hold time at 800.degree. C. and above to a minimum consistent with effective fusion and mixing of the constituent compounds to form high optical quality glass, the composition of which is virtually identical to the composition of the starting material batch. Minimizing loss of volatiles also avoids obstructed viewing of the glass melt, debris drop-in during the hold or on cooling and solidification, and the need for extensive cleanup between glassmaking operations. While the conventional method represents a simple and straightforward approach to glass preparation, it has encountered difficulties in reproducing physical and optical properties from batch to batch. Moreover, such "conventionally" prepared samples often contain inclusions, crystallities and/or flow striae, the latter formed during casting of the melt. U.S. Pat. No. 4,666,486 is incorporated by reference.